The retreading of tires, particularly truck tires, with a pre-cured tread is a widely-practiced art. In such an operation, the tire casing to be processed is prepared by grinding or buffing off any old tread remaining on the casing. Cement is then applied and a layer of uncured cushion stock is applied to the cemented casing. A pre-cured tread layer that has had cement applied thereto is then applied over the cushion stock. Finally, the entire composite uncured retread is inserted into a protective covering or "envelope" for encasing the exterior surfaces of the uncured retread and that retread is cured. The principal purpose of the protective cover or envelope is to prevent air and/or steam used during the curing process from coming into contact with the cement and uncured cushion stock during the curing process. A further purpose of the envelope is to compress the various components described above into tight engagement with the casing so that a suitable product is produced during the curing operation. After the protective cover or envelope is placed around the composite uncured retread, the encased retread is then conventionally placed into a curing chamber or module, typically at a temperature of 200.degree. to 325.degree. F., in an atmosphere at a pressure of e.g. 50 to 150 pounds per square inch. Following the curing process, the encased retread is withdrawn from the curing chamber or module and the envelope is removed from it.
Over the years, several types of retread covers or envelopes have been developed for use in the above-described process. One such prior art product used in this process has been constructed from elastomeric calendar sheet. Two sheets of the material to be utilized in constructing this envelope are cut in equally-sized rings and then seamed together around their outer edges in order to produce a tire cover of the desired shape. Since the sheet goods are necessarily seamed together prior to the curing of the sheet stock, problems have arisen in producing a reliable seam along the outer edges while preventing the more interior portions of the calendar sheets from sticking to each other. The curing process mentioned in the preceding sentence, which is referred to in relation to the making of the protective tire cover or envelope, is not the same curing process which is applied (later) in producing a cure between the pre-cured tread, cement, cushion stock and tire casing during the retreading process, i.e. it is the curing process which is necessary to cure the calendar sheet itself and to produce the cured seam between layers of the sheeting prior to using the completed protective cover or envelope in retreading a tire.
There have been numerous efforts to cure the calendar sheet to satisfactorily produce a good, even circumferential seam around the outer edge while preventing the inner portions of the layers from sticking to each other. For example, it has been proposed to introduce a dry lubricant powder between the portions which are not to be joined while maintaining the seaming surfaces free from that powder so that they may be joined during the curing process. The cover is then cured in a flat configuration. This particular approach has the drawback that it tends to produce weak seams when powder is inadvertently present between the seaming surfaces. Irregular covers are produced in those instances where the inner portions of the sheeting are inadequately coated with the lubricant powder, and therefore, the layers stick to each other during the curing process. Thus, it is generally very difficult to produce a strong, precise circular seam around the outermost edge of the calendar goods to be joined. Furthermore, it has been found that cold checking occurs and air blisters which may form further reduce the durability of the contemplated product.
Presti U.S. Pat. No. 4,036,271 proposes a seamless envelope which is compression formed from an uncured elastomeric slug, using compressive forces of the order of 300 to 600 tons. The disadvantages of the seamed calendar sheets are apparently avoided but among the drawbacks of this process is the fact that heavy, expensive equipment is required.
In accordance with another proposal a butt-seamed or "spliced" tube, very similar to a tire tube, is slit along its innermost radial periphery or circumference. This particular type of cover, while generally satisfactory, does not properly conform to the outer surfaces of a composite uncured retread, and its structure, with its wholly circular cross-section, interferes with its being properly tight in the shoulder area of the tire being retreaded.
It is an object of the present invention to provide an improved retread curing envelope of the butt-seamed type which avoids the drawbacks and disadvantages of prior art proposals.
It is another object of the invention to provide a process for producing a retreading envelope of the character indicated.
It is a further object of the invention to provide a retread envelope which has increased life in comparison with butt-seamed envelopes heretofore produced.
It is still further object of the invention to provide an efficient, inexpensive process for making an improved butt-seamed retreading envelope.